Unaudited Quarterly Data - Schedule Of Unaudited Quarterly Data (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Quarterly Financial Data [Abstract]
Operating revenues	$ 902	$ 1,013	$ 882	$ 813	$ 850	$ 976	$ 853	$ 761	$ 3,610		$ 3,440		$ 3,357
Operating income	224	406	302	238	248	335	302	217	1,170		1,102		1,253
Net income attributable to common shareholders	$ 98	$ 240	$ 170	$ 113	$ 112	$ 187	$ 162	$ 106	$ 621		$ 567		$ 426
Basic earnings per share:
Net income attributable to common shareholders (USD per share)	$ 0.54	$ 1.33	$ 0.94	$ 0.62	$ 0.62	$ 1.04	$ 0.90	$ 0.60	$ 3.44	[1]	$ 3.16	[1]	$ 2.39	[1]
Diluted earnings per share:
Net income attributable to common shareholders (USD per share)	$ 0.54	$ 1.33	$ 0.94	$ 0.62	$ 0.62	$ 1.04	$ 0.91	$ 0.59	$ 3.43	[1]	$ 3.15	[1]	$ 2.38	[1]

[1] Amounts may not calculate due to rounding.